It seems that fields are simply plugins of type
fields. I simply copied the
text folder from /plugins/fields, to a new folder, text2. I also changed the names of the files inside the
text-folder to text2.php and text.xml, and /tmpl/text2.php. In text2.xml there is a reference to the plugin name, update this as well, like:
Now you are ready to use the discover - tool in the joomla administrator, to discover the new plugin. Go to yoursite/administrator/ and find the menu extensions/manage/discover. Click discover, the new plugin should be listed. Install the new plugin. Afterwards, find the new plugin in extensions/mange, and enable it.
Now you should be able to add a new custom field for eg. an article, using your brand new, custom, self made field...
There is a lot of info about writing your own plugin, eg: